The present invention relates to a method for producing a glass cloth suitable to be used as reinforcing materials for making a resin laminate reinforced with glass cloth.
A glass cloth is made by weaving glass yarns treated with a sizing agent comprising starch as a main component of a film forming agent for satisfying processability in the respective steps of spinning, twisting, warping, and weaving. When glass cloth treated with such sizing agent, mainly composed of starch, is used as a reinforcing material for making resin laminates, the starch is usually removed from the glass cloth after weaving because starch gives an adverse effect on the adhesion between matrix resins such as the polyester resin and epoxy resin and the glass fibers constituting the glass cloth.
As a method for removing the starch type sizing agent, namely by desizing or cleaning, there has been proposed a water washing method using surface active agent, enzyme and the like (Japanese Patent Kokai Nos. 63-107847 and 63-108035) and a heating method. However, it is difficult to completely remove the starch type sizing agent by desizing by water washing and at present, desizing is mostly carried out by heating. The method of desizing by heating is a batch type method which comprises heating a rolled glass cloth (roll) at 400.degree.-500.degree. C. for about 30-50 hours in a heating furnace to burn off the starch type sizing agent.
The desizing by heating which is generally carried out is of batch type cannot be continuously conducted. Further, since the time of one cycle is long, the number of rolls used in one cycle must be increased. Thus, a large heating furnace is required and a space for preparation of rolls used for the next cycle is necessary. Furthermore, since heating at high temperature is carried out for a long time, strength of glass cloth is deteriorated and the cost for energy increases.
In an attempt to omit the desizing step, a sizing treatment has been proposed so that the glass cloth after weaving can be used as it is for molding. That is, this method is based on the idea that when a sizing agent comprising an urethane emulsion or an epoxy emulsion is used as the main component of the film forming agent. The resulting glass cloth is molded together with a matrix resin. Thus, the molding can be satisfactorily performed because the emulsion is not as poor in compatibility with the resin as the starch is. However, this method also has the problem in that process-ability in the respective steps of from spinning to weaving is not satisfactory and that impregnation of the resin is insufficient. That is, viscousness of the film forming agent cannot be inhibited and the film forming agent adheres to the traveller and reed in the twisting and weaving steps to cause fluffing or breaking of the yarns. Moreover, if impregnation is not sufficient, the reinforcing effect is not exhibited and the properties of the laminate after molding are inferior.
The above method of desizing by heating is especially not suitable for glass cloth made by weaving high-silica glass yarns containing more than 60% by weight of silica (SiO.sub.2). Glass fibers used at present for making glass yarns of more than 60% by weight in silica content include the three kinds of glass fibers (D-glass, S-glass and Q-glass) as shown in the following Table 1.
TABLE 1 ______________________________________ E-glass D-glass S-glass Q-glass (%) by (%) by (%) by (%) by weight weight weight weight ______________________________________ SiO.sub.2 52-56 73.0 65.0 99.0 Al.sub.2 O.sub.3 12-16 2.0 25.0 -- CaO 16-25 -- -- -- MgO 0-6 -- 10.0 -- B.sub.2 O.sub.3 8-13 22.0 -- -- R.sub.2 O 0-3 3.0 -- -- ______________________________________
E-glass fiber is a non-alkali glass fiber and is used for electrical insulation. D-glass fiber is a low-dielectric glass fiber and is used as substrate for a printed circuit board which requires a low dielectric. S-glass fiber is a high-modulus and high-strength glass fiber and is used as reinforcing materials for construction. Q-glass fiber (quartz glass fiber) is at least 99% by weight in silica content, excellent in heat resistance, and has a low thermal expansion coefficient Q-class fiber is increasingly utilized as a substrate for a printed circuit board for special use.
When glass cloth made from yarns of the three kinds of glass fibers of at least 60% in silica content is desized by heating, the following problems have occurred. That is, D-glass cloth cannot be completely desized by heating. Besides, a laminated sheet made by using the thus desized D-glass cloth becomes yellowish. Moreover, as can be seen from Table 1, D-glass has a relatively high B.sub.2 O.sub.3 component and so there is the problem that when it is heated for a long time of 30-50 hours at 400.degree.-500.degree. C., which are the conditions for desizing by heating, a considerable amount of the B.sub.2 O.sub.3 component is evaporated. Therefore, the glass composition formed after desizing by heating differs from the composition present before subjected to desizing. In the case of S-glass cloth, strength of the glass cloth decreases owing to the heating for desizing as shown in Table 2.
TABLE 2 ______________________________________ S-glass cloth E-glass cloth Warp Weft Warp Weft ______________________________________ Tensile strength 101.5 90.3 83.1 69.2 of cloth as woven (Kg/25 mm) Tensile strength 35.1 31.7 21.0 20.0 of cloth after desizing by heating (Kg/25 mm) Tensile strength 58.0 56.4 34.6 33.4 of cloth after surface treatment (Kg/25 mm) ______________________________________ Note: (1) Sglass cloth: WTA 116E (manufactured by Nitto Boseki Co., Ltd.) (2) Eglass cloth: WEA 116E (manufactured by Nitto Boseki Co., Ltd.)
Table 2 shows the change of tensile strength of the glass cloth. Tensile strength of original cloth as woven to about 30% by subjecting it to desizing by heating. The reduced tensile strength of this original cloth returns to 40-60% of that of the original cloth as woven, by subjecting it to surface treatment. However, the effect of desizing by heating on the tensile strength of the glass cloth is high. Especially, although deterioration of S-glass cloth by heating is smaller than deterioration of E-glass cloth by heating, since S-glass cloth is used because of its high strength, the deterioration of this strength makes it meaningless to use S-glass cloth. In the case of Q-glass cloth, this is wound around an iron core for desizing by heating and put in a furnance for desizing. The thermal expansion coefficient of the Q-glass cloth wound around the iron core is smaller than that of the iron core and, hence, tearings are generated in the cloth. Therefore, employment of desizing by heating is especially difficult for desizing of high-silica glass cloth of 60% or more in silica content.